FIG. 1 illustrates a typical RFID system comprising of a plurality of RFID tags 1 affixed to a detection area 3, associated with an interrogator (reader/writer). The detection area 3 is typically configured to operate within a predetermined field, where the field reads tags 1 present within a detection area 3.
When RFID tags 1 are affixed to items such as books, envelopes, casino chips, and so on, problems occur when tags 1 overlap one another. In such cases, communication performance is degraded significantly and the tags cannot effectively access to the interrogator (reader/writer) 2. Overlaps typically occur when RFID tags 1 stack on one another (denoted by the dotted line in FIG. 1), or when spacing between various tags 1 is set to be very small.
In such cases, the inductance of the resonance coil (Ls) of certain tags 1 interconnect with each other to form mutual inductance, and the inductances of the tags 1 link with the mutual inductance to become exceedingly large. Since resonance capacitors (not shown) present in the tags 1 assume a fixed value, the resonance frequency (i.e., expected resonance frequency and the frequency prescribed by the interrogator) decreases as compared with those in a state free of interconnection.
Furthermore, oscillation is performed on a side of the interrogator (reader/writer) 2 at the same frequency as the expected resonance frequency of the tags 1. Any off-resonance magnetic fields from the side of the interrogator (reader/writer) 2 would then be supplied to the tags 1. In turn, the resonance point of the tags 1 would be decreased due to the overlapping, and the supplied energy is correspondingly decreased. As a result, communication range is decreased.
In order to eliminate a decrease in communication range due to overlapping of the tags 1, JP-A-2000-151480 discloses an identification system where a plurality of tags overlap one another. The document describes a configuration in which capacitors, housed in tags, are switched over to be able to modify a resonance frequency. Furthermore, a method is described in which an internal capacitor switches over for every tag among the plurality of overlapped tags to perform adjustment to a desired frequency. Resonance frequencies of other tags are sequentially modified after the completion of communication with one tag.
Under this method, a resonance frequency of a certain tag can be conformed to a resonance frequency issued by an interrogator, but resonance frequencies of other tags are offset from the resonance frequency issued by the interrogator. Accordingly, a general collision preventive procedure (e.g., JIS 6323-3 ISO/IEC15693-3, or the like) is not used, and interrogator and tags perform communication in 1 to 1.
In addition, there is no method of determining the sequence of a plurality of tags, which tag adjustment of resonance frequency should be performed from, and there is caused a problem that a stable operation is difficult to be ensured in a practical system and so difficult to realize.